1. Field of the Invention
This invention relates to en exhaust purification device for an engine and, more particularly, to an exhaust purification device for an engine provided with a Selective Catalytic Reduction catalyst having a function for purifying NOx.
2. Description of the Related Art
As an exhaust purification device arranged in the exhaust system of en engine such as a diesel engine or the like, there is generally known an exhaust purification device including a NOx catalyst for purifying nitrogen oxides (NOx) included in the exhausted gases. As the Nox catalyst, there is well known a catalyst that selectively reduces and removes NOx by addition of a reducing agent (SCR (Selective Catalytic Reduction) catalyst). Ammonia is known as the reducing agent. In general, aqueous urea (Urea Solution) is injected into an exhaust passage upstream of an exhaust passage section in which the catalyst is arranged. Ammonia is produced from the aqueous urea by receiving heat from the exhaust gas or the catalyst. Then, the NOx is reduced on the catalyst by the ammonia. Such a NOx purification system intended for NOx purification can be called a SCR catalyst system.
On the other hand, Japanese Patent Laid-Open No. 2001-525902 discloses a device in which a module including a gas discharge section and a module including a catalyst section are lined up in order in the direction of the flow of the exhaust gas to treat the exhaust gas. According to an embodiment in Japanese Patent Laid-Open No. 2001-525902, the module including the catalyst section includes a SCR catalyst, and a reducing agent is added to the modulo.
Since the device disclosed in Japanese Patent Laid-Open No. 2001-525902 is provided with the module including the gas discharge section, it requires a discharge device including a battery for use in electrical discharge or a power generator. Therefore, the device has a significant problem regarding energy consumption in a vehicle, and also has a need of taking measures to address an increase in cost associated with this.
Therefore, the present invention has been made in view of the above circumstances and it is an object of the invention to provide a technique of appropriately treating NOx in the exhaust gas without the need for much energy.